drm/dp/mst: Clear port->pdt when tearing down the i2c adapter

BugLink: http://bugs.launchpad.net/bugs/1642572

commit 36e3fa6a upstream.

The i2c adapter is only relevant for some peer device types, so
let's clear the pdt if it's still the same as the old_pdt when we
tear down the i2c adapter.

I don't really like this design pattern of updating port->whatever
before doing the accompanying changes and passing around old_whatever
to figure stuff out. Would make much more sense to me to the pass the
new value around and only update the port->whatever when things are
consistent. But let's try to work with what we have right now.

Quoting a follow-up from Ville:

"And naturally I forgot to amend the commit message w.r.t. this guy
[the change in drm_dp_destroy_port].  We don't really need to do this
here, but I figured I'd try to be a bit more consistent by having it,
just to avoid accidental mistakes if/when someone changes this stuff
again later."

v2: Clear port->pdt in the caller, if needed (Daniel)

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Carlos Santa <carlos.santa@intel.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Tested-by: Carlos Santa <carlos.santa@intel.com>
Tested-by: Kirill A. Shutemov <kirill@shutemov.name>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97666Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1477488633-16544-1-git-send-email-ville.syrjala@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -914,6 +914,7 @@ static void drm_dp_destroy_port(struct kref *kref)
 		/* no need to clean up vcpi
 		 * as if we have no connector we never setup a vcpi */
 		drm_dp_port_teardown_pdt(port, port->pdt);
+		port->pdt = DP_PEER_DEVICE_NONE;
 	}
 	kfree(port);
 }
@@ -2904,6 +2905,7 @@ static void drm_dp_destroy_connector_work(struct work_struct *work)
 		mgr->cbs->destroy_connector(mgr, port->connector);
 
 		drm_dp_port_teardown_pdt(port, port->pdt);
+		port->pdt = DP_PEER_DEVICE_NONE;
 
 		if (!port->input && port->vcpi.vcpi > 0) {
 			drm_dp_mst_reset_vcpi_slots(mgr, port);